This invention relates to gas turbine engines and particularly to the variable stator vanes of its compressor.
Typically, the power plant includes a row of stator vanes ahead of each row of compressor blades to direct the air into the compressor at an optimum angle of attack. In the more sophisticated engines the vane angles vary over the engines operating envelope in an attempt to optimize performance for both design and off-design conditions. With the advent of the higher pressure ratios the controls for scheduling the stator blade angle has become more demanding. Ideally, it is desirable to design the vane schedule so they are flutter free and have better stall margins.
As is well known the angle of attack and aerodynamic characteristics vary from stage to stage of the compressor and that the rate of change of vane angle must be different for each row of vanes. Such systems achieving this feat are exemplified in U.S. Pat. Nos. 2,933,234 and 2,999,630 granted to R. E. Warren, L. V. C. Jensen, G. Neumann, and F. E. Nagel on Sept. 12, 1961 and to G. Neumann on Apr. 19, 1960, respectively. In each of these systems however, the vane actuator stroke versus vane stagger angle are linear yet this results in a nonlinear curve of stroke versus speed and within the confines of the linkage design they are unable to suitably match the stroke versus speed schedule.
I have found that I can achieve a close matched schedule that goes beyond the capabilities of a bell crank and linkage system or similar ones as for example four bar linkage system by selectively adjusting the stroke versus speed characteristics as a function of speed of the compressor. This system requires that the linkage system be first designed such that the vane angle for both the maximum and minimum stroke position is accurate. The vane angle versus stroke position is then plotted for this linkage by using a linear speed/stroke relationship. Then, for a given stroke and vane angle position the correct engine speed is assigned thereby generating an optimum nonlinear speed/stroke relationship for this selected linkage.